1. Field of the Invention
The present invention relates generally to disc loading apparatus and, more particularly, is directed to a mechanism of a disc loading apparatus which ensures that a disc such as an optical disc is clamped on a disc turntable positively.
2. Description of the Prior Art
A magnetic clamping system is known as one of the disc clamping systems utilized in the existing optical disc recording and/or reproducing apparatus (see Japanese Patent Laid-Open Gazette No. 63-234443). An example of a prior-art magnetic clamping system will be described with reference to FIGS. 1A and 1B.
Referring to FIGS. 1A and 1B, there is shown a disc D such as an optical disc in which a disc hub 1 is attached to both surfaces of the disc D at its central portion. The disc hubs 1 are each made of a magnetizable metal plate and secured to both surfaces of the disc D by some suitable bonding material such as a double-sided bonding tape and the like. A disc center aperture 2 is formed through the central portion of each of the disc hubs 1. DA designates the aperture of the disc D.
A clamping device 3 is provided to clamp the disc D. This clamping device 3 is comprised of a saucer-shaped turntable 5 attached to a spindle shaft 4 of a drive motor (not shown) and a magnet 6 secured to the bottom portion of the turntable 5. The top portion of the spindle shaft 4 is projected above from the central portion of the turntable 5. The upper end face of the top portion of the spindle shaft 4 is tapered so that it can be engaged with the center aperture 2 of the disc hubs 1 with ease.
When the disc D is translated in the lower direction during the loading operation from the condition that the disc hub 1 of the disc D is opposed to the turntable 5 of the clamping device 3 (see FIG. 1A), the disc hub 1 is magnetically attracted by the magnet 6 of the turntable 5 and the center aperture 2 is engaged with the top portion of the spindle shaft 4, whereby the disc D is clamped on the turntable 5 under the condition that the disc D is held on the turntable 5 (see FIG. 1B).
In the aforementioned magnetic clamping system, when the disc D is loaded on the clamping device 3, even if the center aperture 2 of the disc D and the spindle shaft 4 are not opposed to each other accurately, i.e., they are displaced from each other by a small amount, the center aperture 2 is guided along the tapered surface of the top portion of the spindle shaft 4 by the magnetic attracting force of the magnet 6, resulting in the disc D being positively clamped on the turntable 5.
However, in the event that the center aperture 2 of the disc hubs 1 or the spindle shaft 4 is damaged because of the frequent use, the friction during the clamping is increased so that the center aperture 2 can not be fully attracted to the top portion of the spindle shaft 4 by only the magnetic attracting force of the magnet 6. Consequently, as shown in FIG. 2, a so-called mis-clamping occurs, in which the disc D is skewed and then clamped on the turntable 5 under such an imperfect engagement condition that the edge portion of the center aperture 2 is brought in contact with the tapered surface of the top portion of the spindle shaft 4. In the event that the turntable 5 rotates under such mis-clamping condition, the center aperture 2 of the disc hubs 1 and the spindle shaft 4 will be damaged considerably. Further, a mis-clamping occurs easily, so that the disc D and the spindle shaft 4 can not be used any more.